The long term objective of this research is to investigate the molecular mechanism for the generation of the diversities of immune receptors. The V, (D), and J segments of the Ig and TCR genes are assembled by somatic recombination during the lymphoid differentiation. These recombination processes are mediated by the recombination signal sequences (Rss) and the V (D)J recombinase. To elucidate the molecular mechanisms of the V (D)J recombination, it is essential to characterize lymphoid-specific proteins that bind the Rss. We have cloned the cDNA for a protein, Rc, which is predominantly expressed in thymocytes, by the ability of Rc to bind the Rss. Subsequently, we have shown the Rc binds the kappa B motif and the Ig kappa chain gene enhancer as well. The aim of this project is to investigate the biological role of Rc in V (D)J recombination. The primary structure of Rc will be predicted from its CDNA and possible structural domains will be defined by comparing with p roteins present in the databases. The information may shed light on how Rc can be involved in V (D)J recombination.